As shown in FIG. 1, the conventional designing rule comprises a body "A", and a plurality of gears "B", "C" and "D" in different sizes . The gears are provided with an involute or other small round hole shapes "E", "F" and "G" respectively; from the center of each gear to the small round holes thereof, different distances are set respectively. The gears "B", "C" and "D" can be fitted in the large gear holes "H" and "I" respectively. When a pencil is set in one of small round holes "E", "F" and "G" of the gears "B" , "C" and "D", one of the gears "B", "C" and "D" may be turned within one of the large gear holes "H" and "I" to draw a given pattern. Various patterns can be drawn by using different gears "B", "C" and "D", different large gear holes "H" and "I", and different small round holes "E", "F" and "G".
However, the conventional designing rule has to be furnished with a given number of round sockets "J", "K" and "L" corresponding to that of gears "B", "C" and "D" in different sizes so as to accommodate the gears. Since the round sockets would take a considerable space and area on a rule, some gears would have no sockets to accommodate, if there are more gears to be used with such a designing rule; some of the gears might be missing because of no proper sockets available. Moreover, the round socket is merely used for accommodating a gear without holding the same in place; in other words, the gear in the round socket is subject to falling out, and causing inconvenience to a user.